This invention relates to a method of charging a plurality of batteries sequentially with pulse charging. The batteries charged by the method of this invention are from one to four single rechargeable batteries, or a single battery or plurality of batteries contained in a battery pack.
Prior art methods of fully charging a plurality of batteries include methods of charging all batteries at once, and methods which sequentially fully charge batteries by time-division switching. Methods of charging all batteries at once include methods which connect batteries in series for charging, and methods which connect batteries in parallel for charging. Since the same charging current flows through all batteries connected in series, series connected methods cannot charge all batteries to full charge if there are differences in remaining battery capacity when charging is initiated. This is because batteries with large remaining battery capacity will be over-charged, and batteries with small remaining battery capacity won't reach full charge. In methods which connect a plurality of batteries in parallel, charging current cannot be uniformly distributed to charge all batteries in an ideal fashion. For example, larger currents will flow in low voltage batteries than in high voltage batteries making it impossible to uniformly distribute charging current. Therefore, all batteries cannot be fully charged under ideal conditions.
This drawback can be eliminated by providing special purpose charging circuitry to establish optimum charging current for each battery. However, battery charger cost for this method of charging becomes high. For example, to charge four batteries under ideal conditions four sets of charging power supplies are required.
As a method of charging a plurality of parallel connected batteries with one charging power supply, it is theoretically possible to distribute charging current to each battery in an ideal fashion by connecting current control circuits in series with each battery. However, practical realization is difficult because each current control circuit itself adjusts internal resistance to control battery charging current, and thus has the drawback of high power consumption. It is necessary to design current control circuits able to control high currents and power making parts cost expensive. In addition, since the current control circuits consume wasted power, it is necessary to design the charging power supply to output higher power. Further, large amounts of heat are generated by the current control circuits requiring a large cooling fan and increasing the parts cost.
These drawbacks can be eliminated by a method which charges a plurality of batteries by sequentially switching from one battery to the next. Since this method charges by sequentially switching from a fully charged battery to the next battery, it has the characteristic capability of fully charging a plurality of batteries with a single charging power supply while controlling charging current to ideal values.
This method of charging is described in Japanese Non-examined Patent Publications No. 4-105521 issued on Apr. 7, 1992 and No. 3-164034 issued on Jul. 16, 1991. The method cited in these and other patent applications initially supplies charging current to only the first battery and charges that battery to full charge. Then, after the first battery reaches full charge, charging of the first battery is cut-off and charging current is supplied to only the second battery which is charged to full charge. In this fashion, charging is switched from the first, second, third, and subsequent batteries to fully charge all batteries.
A method of charging a plurality of batteries by switching from one to the next can fully charge batteries sequentially while keeping the charging current small. However, this charging method has the drawback that it takes too much time to fully charge all batteries. If charging current is increased to reduce the time to full charge, battery performance is degraded. This is because the maximum current for acceptable battery charging is limited. If charging is performed extremely rapidly with large currents, battery temperature rises or the battery is adversely affected causing degradation in electrical performance.
As shown in FIG. 1, this drawback can be eliminated with a method of pulse charging a plurality of batteries by sequential time-dimension switching. FIG. 1 shows pulse charging of a first, second, third, and fourth battery by repeated time-division switching through the charging order. Further, FIG. 1 shows the first battery reaching full charge first and the third battery reaching full charge next. Since charging stops for a fully charged battery, charging is cut-off to that battery.
As shown in FIG. 1, a time-division pulse charging method can reduce charging time for a plurality of batteries compared to a method which switches charging after each battery is fully charged. However, since timing is temporarily interrupted when a battery reaches full charge, all batteries cannot be quickly charged under the most ideal conditions.
The present invention was developed to eliminate these types of drawbacks, It is thus a primary object of the present invention to provide a method of charging that can fully charge a plurality of batteries in a shorter time.
It is yet another important object of the present invention to provide a method of charging a plurality of batteries which can rapidly charge all batteries to full charge without causing battery performance degradation.
It is a further important object of the present invention to provide a method of charging a plurality of batteries which can rapidly charge all batteries to full charge with a charging power supply that can be inexpensively manufactured.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.